PSI - Issue 28

Nina Ogoreltceva et al. / Procedia Structural Integrity 28 (2020) 1340–1346 Nina Ogoreltceva et al. / Structural Integrity Procedia 00 (2020) 000–000

1341

2

Keywords: aluminium electrolysis cells; cathode blocks; protective coating; TiB 2

1. Introduction An increase in energy efficiency, lifetime and safety of aluminium electrolysis cell under operation conditions requires appropriate properties of cells’ bottom materials. In reference to S Ø rlie and Øye (2010), where the basic chemical and mechanical failure mechanisms of graphitized carbon bottom have been discussed, considering the desired properties, a wettability of the carbon cathode surface by liquid aluminium is one of the most notable. Thus the molten metal does not wet the surface of the conventional cathode blocks and forms a layer of the molten alumina on the cathode surface, which results in high current loss due to increasing anode-cathode distance and intensification of cathode wear rate in particular for high amperage aluminum reduction cells. Even during a start-up period electrolyte leaks under the liquid metal and impregnates the bottom blocks. Penetration of the electrolyte and metal into cracks and pores of the cathode blocks, as well as formation of aluminium carbide (Al 4 C 3 ), results in the chemical erosion and the mechanical wear of the carbon bottom. As it has been shown by Reny (2000) and more recently by Skybakmoen et al. (2011), Li (2016) and Senanu (2017), during an electrolysis process peculiarities of current density distribution and the metal/bath movements together influence initiation and dissolution of Al 4 C 3 that leads to formation of uneven wear of the cathode blocks. Skybakmoen et al. (2011) and Li (2016) observed a specific wear profile of the bottom cross-section of “W-shape” or “double-W”-shape. The intensive wear is observed in the most defenseless areas with high current densities - at the edge of crust and directly over the blooms. The very non-uniform wear of the aluminium cells with the graphitized cathodes is today limiting the cell life. For this reason, development of new materials for the cathode surface protection represents a great challenge for the aluminium industry. Over the last 30 years a number of protective materials have been developed. Among the refractory hard materials the preferred for industrial implementation are based on titanium diboride (TiB 2 ). The TiB 2 is advanced ceramics, applied as high-temperature coating due to high melting point (3225 °С), low solubility in the molten aluminum cryolite environment, high hardness and wear resistance, thermal and electrical conductivity as it has been indicated by Munro (2000) and, what is the most notable, the TiB 2 fills the requirement of wetting with molten aluminium. Despite the fact that the combination of the properties makes TiB 2 the one and only functional compound of the protective material of the bottom surface, high cost and lack of stability in service limits its industrial implementation. Thus, a local protection of the most defenseless areas of the cathode surface with the composite coating based on the TiB 2 appears as the most promising solution. In the present work the composition of the wettable TiB 2 -based material for the carbon cathode protection and the cost-effective method of the coating deposition to protect the most defenseless areas of the cell bottom are proposed in relation with the cathode degradation mechanisms. The paper presents the experimental data on the development and property evaluation of the TiB 2 -based coating carried out in laboratory and industrial scales. 2. Experimental procedure Three types of the commercial TiB 2 powders with different particles size distributions elaborated by carbothermic synthesis were chosen to develop the protective coating. In this paper powders are designated as TD1, TD2, and TD3. Commercial water-soluble environmentally friendly binder, which represents the sulfated products of the reaction of naphthalene and formaldehyde (coke residue is not less than 30 wt. %) was used in the coating composition. Particles size distribution and specific surface area of the initial powders was determined in dry conditions using a Laser Particle Sizer ANALYSETTE 22 MicroTec Plus (FRITSCH GmbH, Germany) with measuring range of 0.8 - 2000 µm according to ISO 13320:2020. The initial powders and the protective coating phase structures were identified by X-ray diffraction (XRD) using a XRD7000 diffractometer (Shimadzu, Japan) with CuKα radiation on the angular range of 20°-70° with a step of 0.03° and scan speed of 1.5  /min. The powders as well as polished cross-sections of coating specimens were examined by scanning electron microscope (SEM) JSM 6490-LV (JEOL, Japan) and FEG SEM 7001F (JEOL, Japan) equipped with an electron dispersive spectrometer (EDS, Oxford Inc., U.K.) In order to provide uniformity, workability, and sufficient fluidity of the coating composition and appropriate